When crushing a hard material, for instance stone or ore, a crusher having a crushing gap, also called crushing chamber, is frequently utilized, where material is fed in from above and is crushed between two crusher surfaces that are brought toward each other and between which the hard material is crushed. An example of such a crusher is a gyratory crusher, which has a crushing head provided with an inner crushing shell, which head is fastened on a shaft and during operation describes a gyratory motion, and an outer crushing shell surrounding the inner crushing shell. The fed-in material is then crushed in a plurality of steps between the inner and outer shell. An additional example of a crusher of the type mentioned above is a jaw crusher in which a fed-in material is crushed between a fixed first jaw plate and a second jaw plate mounted on a movable jaw, which second jaw plate moves toward the first jaw plate in a reciprocating motion and in a plurality of steps successively crushes a fed-in material.
After a time of operation, crushing gives rise to wearing of the crusher surfaces and an increased distance between them. WO 93/14870 describes a method to compensate for this wear. In the method described in WO 93/14870, the shortest distance between the inner shell and the outer shell is calibrated on a plurality of occasions during the service life of a first pair of shells. Based on the same data, it is possible to predict how this shortest distance will be altered over time for a new pair of shells and to compensate for this alteration so that the shortest distance between the inner and outer shell in said new pair of shells is kept substantially constant during the entire service life of the shells.
However, the above-described method of compensating for wear has the disadvantage that it cannot produce a crushed material having predictable properties during the service life of a pair of shells.